The continuing proliferation of radio frequency sources operating in an ever more congested radio frequency spectrum creates increasing interference to radar system operation. In a sense, what constitutes music to the ears of those operating the other radio frequency sources is noise to one's own system, radio frequency noise, or, simply, RFI. At times the RFI level is so great that it completely obscures or masks the radar echoes produced by one's radar system and reduces the radar system's ability to detect the desired echo signal from amongst the background signals or clutter, effectively reducing radar system sensitivity.
The interfering RF signals which the present invention addresses are principally narrow band, operating essentially at one frequency, either intermittently or continuously, as example, the higher harmonics of a shortwave broadcast station or a police radar, but also encompasses RFI over a band of frequencies, as example, produced by a television station. Although the RFI energy can be very low and might be disregarded in many instances, for radar systems, RFI cannot be disregarded, since radar echoes often are very low in power level. That is especially so when the echoing surface employs modern radar suppression technologies, "stealth" to reduce radar cross section and reflected signal strength.
Wideband radar systems generate pulses of microwave energy, having very short rise and fall times, or square pulse, as variously termed, and look for return of an echo, the signal reflected from a surface. The amplitude of the echo signal may vary from large to very small, depending upon the distance the echo signal travels from a reflecting object and the reflectivity characteristic of that object. To ensure accurate display of that wideband echo, faithfully reproducing the pulse shape, the systems radar receiver, hence, must have a wide band characteristic at least equal to that of the transmitted pulse.
Advanced pulse communications systems may also be wide band in nature. While the station to station distance is usually known in communication systems, the range of those stations, that is, the distance between the communications stations, can be increased, assuming the power level remains fixed, by reducing RF interference.
Such radio frequency interference is not new. Elimination of the adverse effect of RFI in a radar receiver has typically been accomplished by incorporation of notch filters in the receiver. The notch filter is a radio frequency filter that has a sharply defined frequency characteristic that is tuned to the frequency of the offending RFI source. It selects and effectively blocks passage of the RFI through the receiver's amplifier sections. An adverse side effect is that the notch filter also consumes or dissipates a portion of the desired signal's power, effectively reducing receiver sensitivity. Since the notch filter remains permanently in the circuit, the side effect continues, irrespective of whether the offending station is on the air or not.
In modern times many interfering RF sources are present at different frequencies, requiring a corresponding number of notch filters in the receiving system. The notch filter's side effect of dissipating a portion of the desired signal is unfortunately cumulative. As new sources of RFI present themselves, then additional notch filters must be incorporated into the receiver system to block out such added RFI sources. To construct and install those filters is a time consuming and costly task, in addition to the consequent loss of signal strength caused to the desired signal.
Minimization of RFI is also achieved by use of directional antennas. Radar antennas are designed to be highly directional. That directivity minimizes reception of RFI as may propagate to the antenna from other directions, astride the antenna's axis. Nonetheless even such highly directional antennas contain off directional areas of sensitivity, commonly referred to as side lobes. And offending levels of RFI can be received though such side lobes. While side lobes can be minimized it is not possible to eliminate them entirely.
Accordingly, an object of the present invention is to minimize RF interference in radar and other receivers.
Another object is to remove the unwanted interfering RFI produced by all stations transmitting at any given time, without removing a significant amount of the RF signal that one desires to receive.